Conventional boat propulsion units of the above-mentioned type, for example so-called Aquamatic.RTM. drive units, incorporate a carrier screwed to the boat transom. The propeller drive shaft housing is suspended from the carrier by means of a forked bracket which is pivotable about a horizontal transverse axis accommodated in the carrier. A substantially vertical steering axis or spindle is connected to the drive shaft housing and is journalled in the forked bracket. A steering arm cooperates with the spindle. The steering mechanism of the boat, for example a push-pull cable or a servo unit, acts on the steering arm in order to cause its displacement and thereby that of the propeller drive shaft housing. Trimming and tilting of the drive shaft housing is achieved by pivoting the forked bracket upwardly. This is normally carried out with the help of a pair of hydraulic cylinder arrangements, with one hydraulic cylinder acting on each leg of the forked bracket.